1. Field of the Invention
The present invention relates generally to an air-to-fuel ratio control system for an engine and more specifically to an air-to-fuel ratio control system incorporated with an electronically controlled carburetor for prevention of engine overheat.
2. Description of the Prior Art
Air-to-fuel ratio control systems used with an electronically-controlled carburetor are well known. An example of these systems is disclosed in Japan Published Unexamined Patent Application No. 52-129841, entitled Air-to-Fuel Ratio Control System on Engine Closed Loop. In this application, air-to-fuel ratio is controlled to a stoichiometric mixture ratio by actuating an electromagnetic valve provided for a carburetor connected to an engine intake passage, in order to increase or decrease the amount of fuel supplied to the engine. The electromagnetic valve is feedback controlled in response to signals outputted from an oxygen sensor for detecting oxygen concentration in engine exhaust gas. Further, intake air vacuum is detected for correcting air-to-fuel ratio to a rich mixture when engine load is heavy, thus improving engine operating characteristics under heavy engine load.
In the prior-art air-to-fuel ratio control system as described above, however, although engine operating characteristics are improved under a heavy engine load, no consideration is taken for engine operating characteristics of when intake air temperature or the engine coolant temperature is high.
Therefore, even at high temperatures, air-to-fuel ratio is controlled to a stoichiometric mixture value, so that combustion temperature is high in engine combustion chamber. This results in a problem in that exhaust gas temperature rises and additionally the engine is easily overheated.
A more detailed description of the prior-art air-to-fuel ratio control system will be described in more detail under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.